WASHINGTON -- So much for Date Night.After determining that "no competent breeding" had occurred Friday between giant pandas Mei Xiang and Tian Tian, despite the two having been left alone together for a couple hours, scientists and veterinarians at the Smithsonian National Zoo moved quickly Saturday morning to artificially inseminate Mei Xiang.Pandas are able to breed once a year for a few days. And Mei Xiang and Tian Tian have not been successful breeding naturally, said zoo spokeswoman Pamela Baker-Masson. Their only living offspring, Tai Shan, who was born in 2005 and nicknamed Butterstick, was the product of human intervention, and since then scientists and veterinarians have inseminated Mei Xiang five times without success.The artificial semination that took place Saturday was the first attempt to impregnate Mei Xiang since the death of her cub last year.Mei Xiang -- and Tian Tian -- stunned the public and their caretakers in September when Mei Xiang unexpectedly gave birth. Pregnancy can be hard to spot in giant pandas.Six days after the surprise birth, jubilation dissolved into grief when distress calls by Mei Xiang alerted veterinarians to the cub's lifeless body.A necropsy showed that the cub's lungs were not fully formed, which led to liver failure.This year's panda pregnancy watch began Tuesday, when zoo veterinarians noticed elevated estrogen levels in Mei Xiang. To enable breeding, they closed the giant pandas' habitat to visitors to reduce noise and other distractions and left the pair alone together Friday evening. When that didn't work out as hoped, Mei Xiang was put under general anesthesia Saturday morning and inseminated using a combination of fresh and frozen sperm from Tian Tian, zoo officials said.The procedure was performed by Tang Chunxiang, the assistant director and chief veterinarian of the China Conservation and Research Center for the Giant Panda at Wolong. A second insemination attempt was possible Saturday evening, Baker-Masson said.The next step is familiar by now: Watch and wait."We are hopeful that our breeding efforts will be successful this year, and we're encouraged by all the behaviors and hormonal data we've seen so far," said Dave Wildt, head of the Center for Species Survival at the Smithsonian Conservation Biology Institute. "We have an extremely small window of opportunity to perform the procedures, which is why we monitor behavior and hormones so closely."Female giant pandas can produce offspring as late as their early 20s, but most stop in their late teens, Baker-Masson said. Mei Xiang turns 15 in July, and Tian Tian turns 16 in August. They are at the zoo under a $10 million agreement with China that lasts until 2015 to try to help stave off extinction for the estimated 1,600 giant pandas in the world.The popular Panda Cam, which allows the public to observe the animals in their enclosure day and night, was turned off Saturday morning during the procedure.Panda fanatics might have recognized that as a sign to stay away, but many visitors to the zoo on Saturday were unprepared to see a blue sign that read: "Panda House Is Closed."Berkley Sheffield, 7, looked glum as she leaned against the wire fence overlooking the panda enclosure. No pandas anywhere.Berkley had told all her classmates back home in North Carolina that she was going to see the pandas."She was very excited," said her mother, Megan Sheffield.A few determined zoo visitors made their way up to a higher-level viewing area, hoping to catch a glimpse of a panda through the wire fencing and thick bamboo stalks. No such luck.Deanna Porcher, who lives in Ghana, brought her 2 year-old son, Joel, to see Mei Xiang and Tian Tian."I guess this was not the day," she said."That was the only reason we came!" said Kathy Cameron, 39, who was visiting from Indiana with her family. "Most of these other animals we can see other places." She hadn't heard about the artificial insemination. When she saw the dejected expression on her youngest son's face, she scooped him up and tried to explain.She said: "The doctors are helping the panda today."The exhibit is scheduled to reopen today.

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